I did a little (not more than that, really) and it appears if a professional thatching job is done with high quality thatching reeds, not straw, you can expect the roof to last about 60 years. If that was the case, this 600 yr old thatched roof would have been replaced approximately 10 times.
Granted, I am writing this on assumption that the arrow was found in the thatching, I don't know because I am at work and the audio cannot be turned on with this computer. I didn't get to hear what was being said.
Still, one loves a good mystery!